"Heat-Away 640 is a single-part, 100% solids, copper-filled, thermally conductive, silicone grease used in heat transfer applications from –60 to 550 ºF (-51 to 288 ºC). Heat-Away™ 640 is formulated as a thick paste that will not flow after application and the viscosity remains stable over the entire temperature range. The thermal conductivity is 4.68 W/m-K."

FULL STORY @ AREMCO(http://www.aremco.com/news-item/heat-away-640-thermally-conductive-copper-grease-now-available/)